Metallographic polishing



May 15, 9 K. T. GRUBB ETAI.

METALLOGRAPHIC POLISHING Filed Oct. 5, 1958 INVENTORS Zuwifiw BY Mag) g4) %%q d fi vws tates Uite This application is a continuation-in-part of application, Serial No. 635,810, filed January 23, 1957, now abandoned.

This invention relates to the polishing of metal surfaces with diamond dust for metallographic purposes.

The invention is based on theories regarding the manher, in which polishing should be carried out in order to obtain a polished surface as truly representative as possible of the natural structure of the material and at the same time being as free as possible from scratches of a size to be of disturbance when the polished sample is viewed in the microscope, as is desirable in metallographic polishing.

To illustrate the character of polishing desirable for metallographic purposes, a comparison may be made with the industrial polishing methods used in the making of metal articles. The metallographic polishing differs from the so-called lapping process in that the latter is exclusively the result of the cutting effect of the polishing grains, i.e. even with a relatively small microscopic enlargement the polishing scratches resulting from even very small polishing grains, say of particle sizes in the order of 1a, will be clearly visible. Such scratches should be avoided as far as possible in metallographic polishing.

On the other hand, the process known as honing, which is used in the manufacturing of metal articles for the purpose of obtaining extremely smooth surfaces, is not recommendable for metallographic polishing, because honing is on principle based on a smearing of the metal particles, whereby a distortion of the true structure of the metal surface takes place, i.e. the mircroscopic picture does not show the true structure of the sample, but a structure resulting from cold working.

In fact, the only known polishing process, which is absolutely d stortion free and scratch free, is electrolytic polishing, where the metal molecules are removed from the surface of the metal Without any mechanical treatment.

However, electro-polishing is not suitable for use for all metal surfaces. Especially this method fails in the polishing of very heterogeneous materials, such as cast iron and silumin, that contain non-metallic constituents which do not or practically do not take part in the electrolytic process. The same applies to certain slag particles.

There are therefore many cases Where it is still necessary to use the mechanical polishing method for metallographic purposes, while in other cases it is desirable to have a mechanical polishing method available as an alternative to electrolytic polishing.

For this purpose it has been proposed to use various abrasives, among these also soft abrasives such as A1 MgO or the like. However, such soft abrasives have been found to have a too great smearing eifect in many cases, so that the treatment will be similar to honing, and the glossy surface obtained will not be truly representative of the undistorted structure of the metal. Moreover, owing to the relative softness of the abrasive grains, the polishing will consequently take a very long time.

To avoid smearing even of very hard metals and alloys and also to reduce the polishing time, the best polishing grains available are diamond dust. For the reasons exatent plained above with reference to the so-called lapping process it will be understood, however, that the polished surfaces will not be scratch-free, with the exception of very hard materials such as very hard steel, sintered metal carbides etc.

In many cases it has therefore been found necessary in the past to remove the scratches resulting from diamond polishing by a subsequent polishing with a softer polishing material, such as A1 0 or MgO. This, however, is a time consuming and inconvenient complication.

It is the object of the invention to provide methods and means, whereby the polishing properties of diamond dust can be utilized more advantageously, efiiciently and economically than with previously known methods. 7

With this object in mind, according to the invention, use is made of the fact that a polishing cloth, by means of which the polishing is performed, besides serving as a medium for holding the diamond grains also has a smearing eifect on the metal surface subjected to polishing. In accordance with the principles of the invention, this smearing effect should not be so great as to distort the true microscopic structure of the metal surface, but on the other hand should be great enough to remove disturbing scratches resulting from the cutting effect of the diamond grains to a greater or smaller extent. As 'contrasted to the effect of a honing process, the smearing caused by the cloth itself should only result in a submicroscopic displacement of the particles of material in the surface subjected to polishing.

To achieve the object outlined, a suitable balance should be established between the hardness of 'the metal to be polished, the size of the diamond grains used for the polishing and the hardness of the fibres from which the polishing cloth is made. I

Now, when diamond dust is distributed over the surface of a polishing cloth, e.g. by means of a suitable paste in which the diamond dust is suspended in well known manner, some of the grains will penetrate into the polishing cloth, and this process will be continued during use of the polishing cloth. For this reason it is a difficult problem to maintain the balance specified above because the amount of diamond dust in active position on the very surface of the polishing cloth may vary in an uncontrollable manner. The penetration of diamond particles into the interior of the polishing cloth is also, disadvantageous in other respects because the consequence is that when a suitable quantity of diamond powder'is to be present in the very surface, there must be an excess of diamond powder present in a centain depth below the surface. This excess amount of diamond powder will never be put to use, but will be wasted when the'cloth must be discarded after some time owing to the deposition in the cloth of metal particles that have a harmful in-.

sible by avoiding the presence of too large quantities of inactive diamond particles in the polishing cloth. According to the invention a method of polishing metal surfaces for metallographic purposes comprises the step of subjecting a surface to be polished to the polishing action of diamond dust distributed over the area of a thin layer of textile fabric superposed on an impervious layer in fixed, non-displaceable connection therewith.

. powder than hitherto possible.

. a In a polishing cloth of this construction, the diamond vpowder can only penetrateinto the thin surface layer,

and the surface of thecloth can therefore be suitably saturated with a considerably smaller amount of diamond The said impervious layer contributes towards strengthening and re-inforcing the thin surface layer so that the polishing cloth-can be stretched tightly on a polishing disc, and since the surface layer is non-displaceably united with the impervious layer and the diamond powder cannot penetrate through the latter, it will never be capable of proceeding to the 7 space between the underside of the polishing cloth and the surface of the polishing disc.

cause it has been found that when an abrasive is present I -.at this location, it will tend to collect in Waves owing to the inevitable displacements of the polishing cloth relative to the polishing disc, which would again result in irregularities of the active surface of the polishing cloth. According to a preferred embodiment of the invention,

-.said impervious layer is in ,turn superposed on a cushion layer in fixed, non-displaceable connection therewith. In this manner, a polishing cloth can be obtained which has a suitable softness and volume and at'the same time the desirable polishing properties outlined besides the advantage of absorbing'a considerably smaller quantity of diamond dust than previously known polishing cloths. :The invention will now be described in further detail .with reference to the accompanying drawings, in which FIG. 1 is a perspective view of a polishing cloth ac- .cording to one embodiment of the invention, the layers 'of the cloth being partly separated for better illustration,

FIG. 2 illustrates one way of manufacturing a polishing cloth of the type illustrated in FIG. 1,

FIG. 3 is a perspective view of one form of an apparatus. for carrying out the method according to the invention, a ,FIG. 4 a similar view of another form of an apparatus for carrying out the invention,

' FIG. 5 a perspective view of a polishing disc and a clamping ring for use with the apparatus of FIG. 3 or 4, and a i FIG. 6 the polishing disc of FIG. 5 with a polishing V cloth stretched thereon by means of the clamping ring,

In the dnawing,1is.a layer of woven textile fabric referred to as the surface layer-,2 is a layer which is impervious to diamond particles, this layer being referred to .as-stop layer, and 3 is a layer of a woven textile fabric .re'ferred'to as cushion layer. The three layers are united with one another to form a composite sheet of circular configuration adaptedto be stretched on the polishing disc tobedescnbed in the following with reference to FIG- UREss and s.

The surface layer may consist of various materials depending on the material to be polished by means of the cloth. This is easily ascertained by experiments. It is important that the surface layer should not consist of a 7 material which in itself might have a non-desirable smearing effect on'the material to be polished, but on the other vhand a moderate smearing effect is. often desirable to establish the above mentioned balance between the var- 1ous polishing factors. As an example of amaterial that hasbeen found suitable in many cases raw silk may be mentioned. This is available in great strength at a small thickness with consequent small depth of penetration of the diamonddust, preferably the thickness of. a surface This is of advantage be-.

' ethylene.

particles that these begin to have an unfavourable smearing effect on the surfaces to be treated. In this manner a still greater saving can be obtained of the total quantity of diamond powder used for the polishing. The liquid used for reactivation is usually a weak acid such as 5% H01 or H 50 Materials capable of withstanding treatment with an acid of this character will in the following be referred to as acid resistant.

For thevpolishing of relatively soft materials, such as copper and aluminum, it has been found that the smearing eifectof a surface layer of raw silk is sometime greater than desirable. A textile fabric, which, due to its extreme softness and acid resistance, is advantageous for use in such cases, is the so-called zein fibre cloth, and it has been found that this should preferably have a thickness not exceeding 0.3 mm. in order to obtain a suitable balance between the cutting eifect of the diamond particles and the smearing efiect of the fibres of the cloth.

The stop layer may advantageously consist of a therrno plastic material which is united with both said surface layer and said cushion layer by the application of heat and pressure. A layer of this character is suitable for obtaining a high degree of proofness against the penetration of diamond dust and a reliable and undisplaceable union between the various layers. A suitable thermoplastic material for the purpose here in question is poly- Polyethylene has the additional advantage of being able to withstand washing of the composite sheet with an acid, as above mentioned.

If a cushion layer is used, this should preferably also consist of a :textile material capable of withstanding washing with a metal dissolving acid. In other respects the choice of this material is not nearly as critical as that of the surface layer. It has been found that raw silk of a similar thickness as specified for the surface layer is also suitable for use as cushion layer, but other materials such as cotton fabric may also be used, and the thickness of this layer may be somewhat greater, but on the other hand not too great because this may undesirably affect the flatness and smoothness of the surface layer.

Examples of polishing cloths are as follows:

Example 1 Surface layer: Silk cloth, plain weave Warp: Spun silk yarn, metric .count 270/2, ends per inch, Weft: Reeled silk yarn, crepe twist 2 l2/ l4 deniers 105 picks per inch, Grade: 100% A-grade, Weight:

14 mm =50 g./yd.)

. Stop layer: Polyethylene sheet, two-layer, 0.03 mm. each,

soft (low softening temperature) Cushion layer: Same as surface layer.

Example '2 Surface layer: Zein cloth, plain weave,

Warp: 2/ 60 metric count with 780 spinning twist and 760 twisting twist, staplelength 4 inches, 63

picks per inch, 1 A Weft: As warp,

Stop layer: Polyethylene as under Example 1, Cushion layer: Silk as under Example 1.

FIGURE 2 illustrates one way of making a polishing cloth according to the invention. 4 and 5 represent sup- .ply rollers for a woven fabric of raw silk, s and 7 supply rollers for polyethylene sheets, 8, 9 and 10 guiding rollers, 11 a heating roller, which is internally heated so that the surface thereof has a temperature of about C., 12 an infra red heat radiator,,13 and 14 pressure and cooling rollers, and 15 a take-up roller. With polyethylene as stop layer the laminating temperature may advantageously be about 160 C. and the laminating pressure about 2 kgs./cm. (28.5 lbs/square inch). The speed of travel may be about 1.7 yards per minute. These figures have been found suitable in manufacturing a polishing cloth in accordance with the above Example 1.

For the polishing cloth of Example 2 the same method may be used, only the roller 5 would then be a supply roller for zein fibre cloth and the operating speed should preferably be somewhat lower, say about 1 yard per minute.

It is important that the heat and pressure in the laminating process should be sutficient to make the polyethylene adhere firmly to the yarns of the woven fabrics,

but on the other hand the polyethylene must not become softened to such an extent that it will be squeeezed into the meshes of the woven fabrics. It has been found that the operating conditions specified will fulfill these require ments.

The composite sheet collected on the take-up roller is subsequently cut into circular blanks of suitable size.

For carrying out the polishing according to the invention a composite sheet of the character described is preferably stretched firmly on the surface of a circular polishing disc 16 by means of a clamping ring 17 as illustrated in FIGURES 5 and 6. The polishing disc may e.g. consist of hard polyvinyl chloride, and it may be in the form of a separate part that may be detachably supported either on a stationary base member 18 as illustrated in FIGURE 3 or on a rotatable base member or disc 19 as illustrated in FIGURE 4, e.g. by means of pegstnot shown) on the bottom face of the disc fitting into holes 20 of the base member.

Diamond dust is then applied to the polishing cloth preferably by charging the surface of the latter with a paste in which diamond particles are suspended. Care should be taken to use diamond dust of suitable particle size in order to obtain a suitable condition of balance between the cutting efiect of the diamonds and the smear ing effect of the cloth itself in accordance with the principles outlined above. Thus, it has been found that with the polishing cloth according to Example 1, the grain size of the diamond particles should preferably not be'below Lu. On the other hand when using the polishing cloth according to Example 2, the grain size of the diamond particles should not exceed 1;.

In both cases it is desirable to use diamond particles of the greatest possible homogeneity or'in other words the range of particle sizes should preferably be small and the proportion of particle sizes close to the optimum value should be as high as possible. The grading of diamond dust for the purposes of the invention should therefore be made with the greatest possible care.

Also the viscosity of the diamond charged paste is of considerable importance in order to obtain best results.

It has been found that the synthetic resin Garbo-Wax is a very useful medium as the base of diamond pastes for use with the method according to the invention. Moreover, in order to obtain a suitable viscosity during continued polishing it is desirable that the paste should have lubricating properties which may be obtained by dissolving the synthetic resin above specified in ethanol.

We claim:

A metallographic polishing cloth comprising a thin surface layer of woven textile fabric having a thickness of not more than 0.3 mm., a suspension in a paste-like carrier of diamond dust the particles of which are of the order of one micron in size, said suspension being carried on one face of and penetrating into said surface layer, and a stop layer of material impervious to said suspension united to the other face of said surface layer in fixed connection therewith.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES 1 Industrial Diamond Review, volume 5, No. 57, August 1954, pages 169 to 181. 

